1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a hydraulically operated transmission which is mainly used in a hydraulically operated transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
Conventionally, there is known an apparatus in which the pressure of the oil supply to hydraulic engaging elements provided in a hydraulically operated transmission is controlled by an output pressure of solenoid valves with duty control (hereinafter also called duty-controlled solenoid valves), whereby pressure increase characteristics and pressure decrease characteristics of the hydraulic engaging elements at the time of transmission or speed changing are controlled so as not to cause speed change shocks.
In order to prevent the occurrence of the speed change shocks, it is necessary to finely control the hydraulic oil pressure changes in a relatively low-pressure engagement transition region in which the hydraulic engaging elements transfer from a disengaged condition to an engaged condition. For that purpose, it is desirable to lower the input pressure to the solenoid valves to thereby minimize the rate of change of the output pressure to the duty ratio.
As a hydraulic control apparatus to meet this kind of requirements, there is conventionally known an apparatus provided with a hydraulic oil pressure changeover means comprising a pressure reduction valve, and a changeover valve which can be switched between a high-pressure position for inputting a high hydraulic oil pressure to solenoid valves and a low-pressure position for inputting a low hydraulic oil pressure from the pressure reduction valve to the solenoid valves. At the time of speed changing, the changeover valve is switched to the low-pressure position to lower the input pressure to the solenoid valves, and the changeover valve is switched to the high-pressure position after the speed changing has been completed to thereby increase the input pressure to the solenoid valves (see Japanese Published Unexamined Patent Application No. 21063/1990).
In the above-described conventional apparatus, when the changeover valve is restrained to the low-pressure position due to a trouble or failure by the inclusion of foreign matter or the like, the pressure of the hydraulic oil supply to the hydraulic engaging elements remains in the low pressure. As a result, when a transmitting torque increases due, for example, to the torque amplifying function of a fluid torque converter after the completion of speed changing, there will occur a slippage in the hydraulic engaging elements, resulting in a poor drivability (or a poor feeling to the driver of the vehicle).
In view of the above-described points, the present invention has an object of providing a hydraulic control apparatus for a hydraulically operated transmission in which the oil supply pressure (or the pressure of the hydraulic oil supply) to the hydraulic engaging elements is made high even if the apparatus fails or is in trouble, thereby securing a fail-safe operation.